1. Field of the Invention
This invention relates to a controlling method for injection speed and pressure in an injection molding machine. More particularly, the invention relates to said controlling method in which both of the injection speed and injection pressure can be effectively controlled by the use of an improved pressure controlling system without the use of conventional combination of a flow control valve and a relief valve.
2. Description of Prior Art
In the injection molding of molten synthetic resin material, it has been commonly accepted that the injection pressure and injection speed are significant factors for the injection operation as well as the cylinder temperature, screw rotation speed, back pressure and mold temperature, for a certain molding material and a mold.
In the conventional art, the injection pressure is controlled by the pressure control valve in the hydraulic pressure circuit of the machine, and the injection speed is controlled by the flow control valve. One should note the relation between the above two. That is, when the set pressure of the pressure control valve is higher than the pressure which is necessary to inject the material into the mold cavity, the injection pressure during the injection step is lower than the set pressure of the pressure control valve, and the injection speed is kept at the set point of the flow control valve. And when the filling is completed, the injection speed approaches zero, and the injection pressure rises near to the set pressure of the pressure control valve and kept at this pressure.
While, in case the set point of the injection pressure is lower than the pressure which is necessary for the injection, the actual injection pressure can not be increased higher than the set pressure of the pressure control valve, therefore the injection speed becomes lower than the set point of the flow control valve.
The reason why the injection pressure and injection speed do not always coincide with the set points thereof is that the necessary pressure for filling the material into the mold depends upon the flow speed of the material as well as the resistance from the screw cylinder through the nozzle, sprue, runner and gate into the mold cavity which is determined by the configuration and length of the resin passage and the viscosity of the molten resin.
In the prior art, when the injection speed is changed in the injection process, flow control valves according to the number of steps are provided in the circuit, and limit switches in the same number as said valves are also provided so as to be operated in order during the injection. Thus, the stepwise changes of the injection speed can be carried out by interconnecting said flow control valves and limit switches. However, the total discharge of the pump is passed through said circuit, so that the flow control valves and their respective change-over valves must have sufficient capacities to receive the quantities of the passed hydraulic oil.
As disclosed in the above, the speed control of the conventional hydraulic cylinder is accomplished by the flow control in which the quantity of the hydraulic oil to the cylinder is regulated.